Could not find a parts breakdown on this site for your unit but did find some info on another site.

I do not think you have a defrost problem. Usually a defrost problem will not fix itself. But I would check it just in case something has gone intermittent. The defrost thermostat would be at the top of the list, sometimes you can actually see that it is shot, it will be bulged out. To do this, when it acts up again remove the cover inside the freezer so you can see the evaporator coils. If they are heavily iced/frosted up then defrost is a possibility. Note they must be heavily coated to the point that the fan cannot pull air through them. Also check that the evaporator fan is running.

Also look at the frost pattern on the coils should be fairly even, if not then it could be a low freon problem. Also doubt this as it would probably effect the temperatures all the time.

When it messes up check to see if the compressor and it's fan are running.
If not try turning your temp control to off and then back to the set temp. If the compressor starts up there is a good chance that the thermostat is shot. Note that the other site said this part has been discontinued.

Is the unit/compressor clicking re: trying to start but cannot. Could be the start relay is shot.

Here is a couple good sites with basic fridge troubleshooting/repair help
http://www.applianceaid.com/refrigerator.html
Refrigerator Repair Guide: How To Fix a Refrigerator - ACME HOW TO.com

Note getting a repairman in when it is running OK will just cost you money, they will have to guess at the cause.